Electrochemical fuel cells are generally known in the art and convert fuel and an oxidant to electricity. One such fuel cell is a solid polymer electrochemical cell and includes a plurality of membrane electrode assemblies (MEA), each of which includes an ion exchange membrane or other electrolyte disposed between an anode and cathode. The MEA may include a catalyst or other catalytic material at each interface between the ion exchange membrane and the anode to induce a desired electrochemical reaction. The electrodes are electrically coupled to provide a circuit for conducting electrons between the anodes and the cathodes through an external circuit.
In a hydrogen powered fuel cell, hydrogen and air are supplied to electrodes on either side of the ion exchange membrane. Hydrogen is typically supplied to the anode where the catalyst promotes a separation into protons and electrons that are conducted through the external circuit. On the opposing side of the membrane, air is provided to the cathode where oxygen in the air reacts with the protons passing through the ion exchange membrane to produce byproduct water.
The hydrogen fuel fluid stream supplied to a fuel cell anode may be, for example, substantially pure hydrogen, or a dilute hydrogen stream such as a reformate stream. Further, the anode exhaust stream containing unreacted hydrogen, or a portion thereof, may be recirculated back to the fuel cell, depending upon a measured concentration of unreacted hydrogen contained in the exhaust stream. It is known to provide hydrogen sensors operably associated with a fuel cell exhaust stream for measuring a concentration of hydrogen in the exhaust stream. Further, the concentration of hydrogen within the exhaust stream may be used as an indicator of the fuel cell performance and operating efficiency. For example, if there is an excessive amount of hydrogen in the fuel stream exhausted from the fuel cell, it may indicate poor operating efficiency.
However, fuel cell exhaust gasses consist of nitrogen, trace hydrogen, liquid water and water vapor at a temperature between 60° C. and 80° C. (140° F. and 176° F.) with nearly 100% relative humidity. This high absolute humidity generates condensate inside the hydrogen sensor, which may result in temporary or permanent incorrect hydrogen concentration readings. Additionally, chemical hydrogen sensors, such as those used in current fuel cell vehicles, generate water vapor by themselves due to a reaction of free hydrogen with oxygen on the surface of the sensor while detecting the hydrogen concentration.
Available hydrogen concentration sensors are designed for ambient application and usage in low humidity environments. It is known that use of available sensors in high humidity environments where condensation and water drops may occur adversely impacts the lifetime and reliability of the sensor. The combination of high temperature and high humidity within the sensor assembly may lead to corrosion or degradation of the sensor or components and wiring thereof, requiring premature and costly replacement of the sensor. Moreover, it has been determined that condensed water inside the hydrogen sensor is a primary reason for diminished reliability and durability of the sensor.
Use of available hydrogen concentration sensors for fuel cell exhaust gas applications does not meet automotive requirements for durability, reliability and cost. Presently, a primary method of addressing the drawbacks of current sensor technology is to provide specially designed sensors that deter corrosion of the sensor. These exhaust sensors, for example, require coated contacts, making the system more expensive. It is therefore desirable to provide an ambient hydrogen sensor that does not corrode, eliminating a cost intensive measure of frequent exchanges or replacements of the hydrogen sensor after a relatively limited number of operational hours. The high frequency of the sensor exchange rate impacts a vehicle's reliability, removes it from service frequently for sensor replacement, and increases the servicing or lifetime costs of the vehicle.
There is a continuing need for a cost effective, long lifetime hydrogen concentration sensor assembly that militates against water vapor condensation and liquid water inside the sensor housing.